A photographic process utilizing a silver halide has heretofore been widely used due to its excellent photographic characteristics such as sensitivity, control of gradation, etc., as compared with other photographic processes, such as an electrophotographic process or a diazo photographic process. In recent years, with respect to image formation processes for light-sensitive materials using a silver halide, a technique capable of easily and quickly forming an image, for example, a dry development process such as a process of using heat, has been developed for the conventional wet development process using a developing solution.
Heat-developable light-sensitive materials are known in the art. Heat-developable light-sensitive materials and processes suitable therefor are disclosed in, for example, Shashin Kogaku no Kiso, Corona Co., Ltd., pages 553-555, Eizo Joho, April 1978, page 40, Nebletts Handbook of Photography and Reprography, 7th Ed., Van Nostrand Reinhold Company, pages 32-33, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, June 1978, pages 9-15 (RD-17029).
Many processes for obtaining color images using a dry process have been proposed.
With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with a coupler, use of a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as disclosed in U.S. Pat. No. 3,531,286, a p-aminophenol type reducing agent as disclosed in U.S. Pat. No. 3,761,270, a sulfonamidophenol type reducing agent as disclosed in Belgian Pat. No. 802,519 and Research Disclosure, pages 31 and 32 (Sept., 1975) and the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler as disclosed in U.S. Pat. No. 4,021,240 has been proposed.
These processes, however, are disadvantageous in that turbid color images are formed since a reduced silver image and a color image are simultaneously formed in the exposed area after heat-development. In order to eliminate this disadvantage, a process which comprises removing a silver image by liquid processing and a process which comprises transferring only the dye to another layer, for example, a sheet having an image receiving layer have been proposed. However, the latter process is not desirable because it is not easy to transfer only the dye as opposed to unreacted substances.
Another process which comprises introducing a nitrogen containing heterocyclic group into a dye, forming a silver salt and releasing a dye by heat-development is described in Research Disclosure, RD-16966, pages 54-58 (May, 1978). With this process, clear images cannot be obtained because it is difficult to control the release of dyes from the non-exposed areas, and thus it is not a generally applicable process.
Further, process for forming a positive-working color image by a silver dye bleach process utilizing heat as well as useful dyes therefor and methods for bleaching are disclosed in, for example, Research Disclosure, RD-14433, pages 30-32 (April, 1976), ibid., RD-15227, pages 14-15 (Dec., 1976) and U.S. Pat. No. 4,235,957.
However, these processes require an additional step and an additional material for accelerating a bleaching of the dyes, for example, a step of heating with a superposed sheet with an activating agent, and also have the disadvantage that the resulting color images are gradually reduced and bleached by free silver which is also present during storage for a long period of time.
Moreover, a process for forming a color image using a leuco dye has been disclosed in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, this process involves a difficulty in incorporation of the leuco dye in the photographic material in a stable manner and causes gradual coloration during storage.
In addition, all of the above-described conventional processes generally require a long period of time for development to obtain images of high fog and low density.